The present invention relates generally to oscillators, and more particularly to improving amplitude control and frequency stability in oscillators which include on-chip MEMS (micro electromechanical) resonators.
The closest prior art includes the article “High performance crystal oscillator circuits” by E. Vittoz et al, JSSC vol. 23, no. 3, June 1988 pp. 774-783. FIG. 11 of this reference is essentially reproduced in subsequently described Prior Art FIG. 1. The techniques disclosed in the Vittoz article are widely used. (In Prior Art FIG. 1, note that the same reference characters are used as in FIG. 11 of the Vittoz article.)
In the high-performance quartz oscillator shown in Prior Art FIG. 1, terminals Q1 and Q2 are connected to a quartz resonator, as indicated by dashed lines. An AC signal V1 is produced on terminal Q1 and a signal V2 is produced on terminal Q2. The AC output current I0 produced by oscillator circuitry including P-channel MOS transistors M1, M15, M17 and M19 and N-channel current mirror output transistors M10 and M2 is mirrored, by means of N-channel transistor M8 of an amplitude regulator circuit, to the drain and gate of a P-channel transistor M9 and to the gates of P-channel transistors M37 and M39 of the amplitude regulator circuit. P-channel transistors M3 and M5 and N-channel transistors M4 and M6, and resistor R7 also are included in the amplitude regulator circuit. An output voltage regulator includes P-channel transistors M21, M25, M27, M33, M29, M33, and M35 and N-channel transistors M22, M20, M24, M26, and M32.
The AC signal generated in the high-performance quartz oscillator of Prior Art FIG. 1 passes through the diode string including transistors M37 and M39, each of which actually functions as a resistor, the resistance of which is determined by its gate voltage, that is, by the voltage across diode-connected P-channel transistors M7 and M9. Transistor M37 provides DC feedback for transistor M3, so that it functions as a peak detector, with capacitor C11 functioning as a filter. The current in transistor M3, which corresponds to the peak value of the input voltage V1 on terminal Q1, is compared to the current through transistor M4 and then is filtered by the low pass circuitry including transistor M39 and capacitors C4 and C5. The result of the above mentioned comparison is to change the current through transistor M5. The current in transistor M5 is mirrored to increase the current through transistor M1 and to increase the oscillation amplitude. Resistor R7 decreases the gain of transistor M5 gain to provide improved loop stability. The AC signal V1 on terminal Q1 passes from the oscillator circuit through capacitors C1 and C7 to the gate of transistor M9. This results in an AC signal at the gate and drain of transistor M9. The input signal V1 on terminal Q1 also passes through capacitor C7 and creates an AC current through transistor M9. The current through transistor M9 is, in effect, compared to the current in current mirror output transistor M4. The RC filter formed by transistor M39 and capacitor C4, along with the regulator currents through transistors M4 and M5, functions to regulate the amplitude of the oscillator signal V1, and consequently the RC filter and the regulator currents through transistors M4 and M5 also regulate the voltage across capacitor C1 and the current through the quartz resonator. (The current through the quartz resonator is equal to the voltage across the capacitors C1 multiplied by the capacitive reactance of capacitor C1.)
The heart of the Pierce oscillator circuit in Prior Art FIG. 1 uses transistor M1 in a grounded source configuration, with the resonator connected between terminals Q1 and Q2. Transistor M1 is biased by current I0 delivered via current mirror output transistor M2 from the amplitude regulator circuit by means of current mirror input transistor M6. Transistor M17 is operated in weak inversion as a resistor that forces transistor M1 into its active mode and therefore must have a very high resistance to avoid degrading the frequency stability and increasing the current. The resistance of transistor M17 is determined by the biasing transistors M15 and M19, which are matched to transistors M1 and M17.
The amplitude regulator circuit in Prior Art FIG. 1 is based on known circuitry in which high-value resistors are implemented by transistors M17 and M39 that are biased by transistors M7 and M9. In the absence of oscillation, the output voltage regulator behaves as a current reference which delivers a start-up current to the oscillator. As the oscillation amplitude increases, the current I0 decreases until the amplitude of the AC voltage V1 at terminal Q1 reaches a value solely determined by the low current gain of the loop including transistors M3-M6, if transistors M3 and M5 are in weak inversion. This amplitude can be adjusted to a higher value by the capacitive divider C7,C11. The output amplifier is a simple CMOS inverter including transistors M13 and M14 and is biased in its active mode by the matched inverter including transistors M11 and M12 and by transistor M23. It is capacitively coupled to terminal Q1. The total gate-to-drain capacitance CM of transistors M13 and M14 causes an input conductance G1 to load the oscillator, which can be evaluated as described in the Vittoz article.
In Prior Art FIG. 1, the regulating signal I0 generated by the amplitude regulator circuit is the biasing current of the single transistor M1 of the Pierce oscillator. That biasing current I0 determines the transconductance of transistor M1 and hence the transconductance of the oscillator circuit, and therefore controls the amplitude of the AC signal V1 being generated.
In quartz oscillators, an excitation voltage signal is generated inside the crystal by the piezo effect, so quartz resonators do not need a separate excitation signal. The amplitude of this piezo-generated excitation signal is so large that small amplitude variations do not cause significant change in frequency. Therefore, the amplitude regulator circuit in Prior Art FIG. 1 provides very crude voltage amplitude control because, for example, the frequency of oscillation will be the same whether the amplitude is equal to 1 volt or 100 millivolts. The oscillation frequency of a quartz oscillator is far less sensitive to amplitude variations than oscillators which include MEMS resonators, and the amplitude regulator circuit of a quartz oscillator therefore needs to provide only enough regulation to keep the circuit in its linear operating range.
Unfortunately, the amplitude control circuitry in the widely used quartz oscillator circuit of Prior FIG. 1 circuit uses translinear relationships in MOS (or bipolar) transistors. Such relationships are highly affected by component matching and temperature. The accuracy of the amplitude regulation of quartz oscillators of the prior art is very poor, typically only tens of millivolts, and is not acceptable for high-performance silicon MEMS oscillators.
Thus, there is an unmet need for a high performance oscillator circuit and method that provide much more precise regulation of oscillator frequency than has been achieved in the prior art.
There also is an unmet need for a high performance MEMS oscillator circuit and method that provide much more precise regulation of oscillator frequency than has been achieved in the prior art.
There also is an unmet need for a high performance MEMS oscillator circuit and method that provide much more precise regulation of oscillator frequency than has been achieved in the prior art, without requiring complex compensation of circuitry for regulating the amplitude of an AC signal produced by the oscillator circuit.